The latest frontrunner in the Republican presidential race, Newt Gingrich, faced a conservative backlash on Wednesday after advocating a "humane" approach to illegal immigrants, a red hot issue for Tea Party activists and other rightwingers.

His proposal to allow most of the country's estimated 12 million illegal immigrants to remain in the US – which he made during Tuesday night's nationally televised GOP debate in Washington – runs counter to Republican orthodoxy, which is opposed to amnesty and in favour of expulsion.

Conservative activists predicted that the comments from Gingrich, who has enjoyed a surge over the last week that has seen him rise to the top of polls of likely Republican voters, will explode in his face.

"I think he will take a hit," said Ryan Rhodes, leader of the Tea Party in Iowa, where the first of the Republican presidential contests is to be held on January 3. "I think that is an issue where he parts company with conservatives."

He added: "He has opened up a chance for someone else to step up again."

The Republican hunt for a candidate to take on Barack Obama for the White House has been volatile. As conservatives search for an alternative to former Massachusetts governor Mitt Romney, regarded as unreliably moderate for their tastes, alternatives have risen and slipped back: congresswoman Michele Bachmann in August, Texas governor Rick Perry in September, former pizza company and chief executive Herman Cain in October. Now it is Gingrich's turn.

The latest poll, from Quinnipiac, has Gingrich on 26%, Romney on 22%, Cain on 14%, Perry on 6%, Ron Paul on 6% and Bachmann on 4%, with Rick Santorum and Jon Huntsman tied at 2%.

The US media was divided over whether Gingrich's views on immigration should be lauded as a rare example of courage and honesty on the part of an American politician or another example of him shooting himself in the foot.

During the debate, Gingrich said: "I do not believe that the people of the United States are going to take people who've been here for a quarter of a century … [and] separate them from their families and expel them."

He added: "I don't believe that the party that says it's the party of the family is going to say it's going to destroy families that have been here for more than a quarter of a century. I'm prepared to take the heat in saying: 'Let's be humane in enforcing the law.'"

His Republican rivals, mainly Romney, Bachmann and Santorum, jumped on the remark, describing it as a mistake. They described Gingrich's plan as amounting to amnesty, that crime should not be rewarded and that the priority should be securing the border.

Illegal immigration is a sensitive political issue. Party members say illegal immigrants have committed a crime and should be deported. But Democrats say that the mass expulsion of the estimated 12 million is not feasible and that anyway it would do enormous economic damage.

Illegal immigrants, the majority from Central and South America, do jobs on farms, in factories and in hotels, and in domestic settings, which many Americans appear unwilling to do.

With Latinos making up the fastest-growing minority in America, Republicans from George Bush to John McCain have backed immigration reform but been forced to retreat in the face of party grassroots hostility.

Speaking in the spin room after the debate, Eric Fehrnstrom, Romney's spokesman, sought to exploit what he viewed as Gingrich's vulnerable spot.

He described it as a mistake. "I think it is a legitimate and major difference between Romney and Gingrich," Fehrnstrom said.

But Gingrich's spokesman, RC Hammond, refused to backtrack, describing his candidate's position as reasonable, and stressing that he had also called for the border to be secured.

Immigration is a big issue in both Iowa and South Carolina, where the Republican primary is to be held on January 21.

It was immigration that led to Perry losing his frontrunner status. In an earlier debate, Perry advocated paying for the children of illegal immigrants to be educated, and said that anyone who did not agree with him did not have a heart. Many Republicans took umbrage at being called heartless.

Rhodes, speaking from Iowa in a phone interview, saw Gingrich's remark about his immigration policy being "humane" as a liberal one, comparable to Perry's remark in its capacity to offend. He said he does not think there is an easy fix on illegal immigration but a blanket amnesty was not the answer.

Rhodes, who is backing Bachmann, estimated that about 60-70% of Republicans likely to vote in the caucus had not yet made up their minds. He is opposed to Romney and predicted he would get no more than 15-20% of the vote in Iowa.

Charlie Gruschow, the chairman of the Tea Party of America, shared the view that Gingrich might be damaged by his remarks. "It could hurt him," he said.

He added: "I like Newt. He has a tremendous mind and is a phenomenal historian. But there are some issues and garbage in his past."

For the party of family values, Gingrich's two affairs and three wives, one of whom he reportedly left while she was ill with cancer, is a black mark.

Gruschow is backing Cain, in spite of allegations of sexual harassment of staff in the 1990s and gaffes on foreign policy.

Mark Meckler, co-founder of the Tea Party Patriots, who is based in California, said Gingrich's views on immigration were well-known and should not have come as a surprise. He felt the vital issue was to firstly to secure the border. "Once that is done, we will be able to have a more mature and less emotional debate," he said.

He has not yet decided who to support. "Frankly, I do not trust any of them. It is a matter of who I distrust the least, who will win the presidency and toe the line once in office," Meckler said.